I don't deny the fluffyness of it, really. Its just a balance thing.
1. You are utterly boned as it is if you get that close to nids.
2. The universal 4+ change makes tyranid escorts even tougher, and it has always bothered me how well armored the nids can be with just organic armor. So you have, already, very tough, very numerous, very deadly nid escorts to kill, you make them harder to hit with a reliable kill tactic that some fleets have, and then if you keep the current 2-dice-take-lowest, they are even more gauranteed to stick around.
I love the way nids play for the most part, but if you have such a hard hitting, cheap, vast swarm, it shouldnt be harder to kill than pricier, adamantine clad space ships.